Add simple styling

Please note this is my first contribution to any public Django codebase, so you may have suggestions for further changes before accepting this request. It's some time since I used Django and I was never heavily into it. I am not necessarily aware of current best practices as I have only jsut started working with django-cms.

Basically I have added another model column, style, set by  drop-down in the editing widget. This is used to set the class of the generated table and select one of a number of styles specified in an auxiliary CSS file.

I have tested the schema migration, which appears to work (I haven't used South before, either).